This invention relates to system clock generators and, more specifically, to multi-frequency system clock generators with errorfree frequency switching.
In microprocessor based electronic systems and in other electronic systems it is often necessary or desirable to use more than one basic clock frequency. The clock frequency is, generally, the frequency which provides clock signals to a number of digital integrated circuits or other digital circuits. One exemplary situation requiring the use of a multi-frequency clock generator occurs in systems in which the main power source is the AC power mains, and which include critical circuitry that must remain in operation and/or store data in an electronic memory should the main power source fail. In such systems, a standby battery is often provided to supply emergency power to time-keeping circuitry and to preserve full memory storage until the main power is restored. Since it is not unusual for the battery to remain inactivated for extended periods of time (with some loss in stored energy) and since it is impossible to predict the duration of the power outage, it is essential that battery power drain be minimized when the battery is activated to provide emergency power. One of the ways to reduce battery drain is to operate the system at a lower clock frequency.
In the past, switchover from one clock frequency to another has been accomplished by means of monostable multivibrator or resistor/capacitor (RC) delay circuits which are relatively complicated and often unreliable. In operation these circuits frequently cause errors in the form of narrow pulses, which cause major system problems, hard-to-diagnose conditions, and testing constraints.
The present invention provides a low cost, low complexity, multi-frequency system clock generator with error-free frequency switching circuitry. As described more fully hereinafter, the circuit comprises two bistable switch circuits, and an RC oscillator made up of a NAND gate circuit, an inverter, and associated RC time-constant circuitry.
These and other features provided by the present invention will be more fully described in the detailed description of the invention.